For many years, a number of medical devices (e.g., pacemakers, vascular grafts, stents, heart valves, etc.) that contact bodily tissue or fluids of living persons or animals have been developed, manufactured and used clinically. A major problem with such articles is that their surfaces tend to adsorb a layer of proteins from tissues and fluids such as tears, urine, lymph fluid, blood, blood products, and other fluids and solids derived from blood. The composition and organization of this adsorbed protein layer is thought to influence, if not control, further biological reactions. Adverse biological reactions such as thrombosis and inflammation may diminish the useful lifetime of many devices.
Implantable medical devices also tend to serve as foci for infection of the body by a number of bacterial species. These device-associated infections are promoted by the tendency of these organisms to adhere to and colonize the surface of the device. Consequently, it has been of great interest to physicians and the medical industry to develop surfaces that are less prone in promoting the adverse biological reactions that typically accompany the implantation of a medical device.
One approach for minimizing undesirable biological reactions associated with medical devices is to attach various biomolecules to their surfaces for the attachment and growth of a cell layer which the body will accept. Biomolecules such as growth factors, cell attachment proteins and cell attachment peptides have been used for this purpose. In addition, biomolecules such as antithrombogenics, antiplatelets, anti-inflammatories, antimicrobials and the like have also been used to minimize adverse biomaterial-associated reactions.
A number of approaches have been suggested to attach such biomolecules. These approaches typically require the use of coupling agents such as glutaraldehyde, cyanogen bromide, p-benzoquinone, succinic anhydrides, carbodiimides, diisocyanates, ethyl chloroformate, dipyridyl disulphide, epichlorohydrin, azides, among others, which serve as attachment vehicles for coupling of biomolecules to biomaterial surfaces. For example, covalent attachment of biomolecules using water soluble carbodiimides is described by Hoffman et al., "Covalent Binding of Biomolecules to Radiation-Grafted Hydrogels on Inert Polymer Surfaces", Trans. Am. Soc. Artif. Intern. Organs, 18, 10-18 (1972); and Ito et al., "Materials for Enhancing Cell Adhesion by Immobilization of Cell-Adhesive Peptide", J. of Biomed. Mat. Res., 25, 1325-1337 (1991).
One type of biomolecule which is often coupled to biomaterial surfaces with coupling molecules is protein. Proteins are polypeptides made up of amino acid residues. A protein comprising two or more polypeptide chains is an oligomeric protein. In general, established coupling procedures couple proteins to substrate surfaces through a protein's lysine amino acid residues which contain terminal amino groups. This method of binding has several inherent problems. For example, if a number of lysine residues are present on a protein's surface, multiple attachments may occur. Multiple attachment sites may lead to multiple conformations of the protein on the biomaterial surface. The lack of coupling specificity may disrupt or destroy the biological activity of the protein being coupled. In addition, coupling molecules may add instability to the biomaterial surface and increase the prospect for burial of the attached protein in the coupling layer. Coupling molecules may also create nonspecific and undesirable crosslinks between protein molecules, thereby destroying the biological properties of the protein or they may create bonds amongst surface functional sites, thereby inhibiting attachment. The use of coupling molecules may also decrease the specificity for attachment of the protein to the biomaterial surface, thereby losing conformational control over the attachment process.